Fuel injection system comprising an injection valve for a self-igniting internal combustion engine

ABSTRACT

A fuel injection system which includes an injection valve for a self-igniting internal combustion engine with direct fuel injection, in particular for liquefied gas fuels, and which is configured as a pintle-type nozzle opening against the force of at least one spring, has the opening force of its fuel needle directly provided by the magnetic force of a solenoid so as to create simplified rate shaping. The magnetic force increases quadratically with the needle lift vis-a-vis the force of the spring increasing linearly with the needle lift, resulting in an opening velocity of the needle increasing approximately quadratically, at least in certain regions, thus permitting small pintle lifts, and the needle has a maximum needle lift of smaller than 1 mm, i.e., preferably smaller than 0.5 mm.

BACKGROUND OF THE INVENTION

The invention relates to a fuel injection system which includes aninjection valve for a self-igniting internal combustion engine withdirect fuel injection, in particular for liquefied gases, which isconfigured as a pintle-type nozzle opening against the force of at leastone spring and permitting a throttled fuel flow during part of the liftof the fuel needle due to the stepwise opening of the nozzlecross-section during the needle lift.

DESCRIPTION OF THE PRIOR ART

Fuel injection systems for diesel engines have hydraulically operatedfuel needles, i.e., a pump is used to apply fuel pressure, which willopen the needle against the force of a spring once the needle openingpressure has been surpassed. In some systems the point in time when thefuel pressure is applied on the needle may be chosen by opening asolenoid valve. Direct operation of the fuel needle by the solenoidvalve is not employed in diesel injection systems, as the injectionpressures permissible with solenoids are not high enough for mixtureformation in diesel engines.

For rate shaping in diesel engines with indirect fuel injectionsingle-hole pintle-type nozzles are used. The pintle will check the fuelflow in the nozzle and reduce the rate of flow compared to the fullyopened nozzle, so that combustion will be initiated less sharply. With ahydraulically operated fuel needle the opening velocity is approximatelylinear. During the lifting of the pintle the flow of the injected fuelis reduced. After this there is a strong increase in flow. Thethrottling characteristic thus primarily depends on the pintle lift andthe geometry of the pintle.

In diesel engines with direct fuel injection dual-spring nozzle holdersare employed, where the fuel needle overcomes the force of two springsplaced one behind the other. As a consequence, the characteristic of theneedle opening velocity is nonlinear. To improve mixture formationmulti-hole nozzles are used in such instances.

Other known fuel injection systems are used in Otto engines with directinjection, in which the fuel needle is directly opened by a solenoidvalve against the force of a spring. This is made possible by thecomparatively low injection pressures required for Otto engines ascompared to those which are necessary for diesel engines. Since in Ottoengines with direct fuel injection the injection point usually is faradvanced relative to the ignition point, rate shaping would not achieveits aim and is not provided.

For special liquefied gases which are burnt in engines with direct fuelinjection based on the diesel principle, the use of rate shaping isdesirable and of advantage, however. Although it would be possible toemploy rate shaping with a hydraulically operated fuel needle as knownfrom the diesel engine, the complexity and expense involved in itsconstruction would be relatively high. In diesel engines this expense isjustified on account of the high injection pressures encountered.

SUMMARY OF THE INVENTION

It is an object of the invention to develop a fuel injection system withrate shaping for the injection of liquefied gas, whose construction iskept as simple as possible.

In the invention this is achieved by providing that the opening force ofthe needle be directly provided by the magnetic force of a solenoid,where the magnetic force increases quadratically with the needle lift asopposed to the force of the spring increasing linearly with the needlelift, will result in an opening velocity of the needle increasingapproximately quadratically, at least in certain regions, thuspermitting small pintle lifts, and where the needle has a maximum needlelift of smaller than 1 mm, i.e., preferably smaller than 0.5 mm. Thesmall initial velocity of the needle will result in a long period ofreduced flow, in spite of the small pintle lift. In this way the needlelifts may be considerably reduced, compared to their normal height ofabout 1 mm in pintle-type nozzles which is required to obtain a flowthat is sufficiently reduced. Due to the combined action of magneticforce and spring force the opening velocity up to the maximum needlelift may increase by a factor of ten and more. A very small openingvelocity may be obtained by providing that the characteristic of thespring force in the throttled region be approximately parallel to thetangent to the characteristic of the magnetic force at the closing pointof the needle.

In view of the low injection pressures possible with liquefied gases,the fuel needle may be directly operated by a solenoid. The mainadvantage of using a fuel needle that is directly operated by a solenoidis that leakages along the guide of the needle are avoided and that nopressure relief is required for the space behind the needle. It is goodto avoid such unloading since the pressure level prevailing in systemsrunning on liquefied gas is at least that of vapor pressure, unlike theatmospheric pressure prevailing in diesel engines. With a hydraulicallyoperated injection valve there would be a strong drop in efficiencysince the opening pressure would have to overcome the pressure behindthe fuel needle in addition to the force of the spring.

Like in diesel engines, fuel injection takes place just before ignition,the latter being initiated when the self-ignition temperature isexceeded. For this reason rate shaping is essential to obtain lownitrogen oxide emissions and quiet running of the engine. The system ofthe invention thus exhibits the simple design of fuel injection systemsused in Otto engines in addition to effecting rate shaping in a simplemanner.

It is proposed in a very advantageous variant of the invention that thespring force acting on the fuel needle be provided by two seriallyplaced springs with different characteristics. The fuel needle pressesagainst the two springs located one behind the other, of which thefirst, weaker spring opens even if the solenoid forces applied are weak.The second spring, which joins the first spring when the lift permittedby the first spring is surpassed, opens only if strong solenoid forcesare applied. After the first spring has opened a certain length of timeis required until the solenoid force rises to the value needed to openthe second spring, during which time the pintle will remain in thethrottling region.

It is provided in a preferred variant that the electromagnet have atleast two solenoids to be activated independently of each other forcontrol of the magnetic force acting on the fuel needle. The fuel needleis operated by the two solenoids which are connected in series. Thefirst solenoid will apply a certain force which will open the needleuntil the throttling position is reached. When the second solenoid isadditionally energized the solenoid force will rise so that the nozzlewill open completely. By suitably choosing the activation times of thesolenoids the timing of the reduced flow may be adjusted as desired. Tosupport stepped movement of the fuel needle serially placed springs maybe added.

In another preferred variant of the invention the proposal is putforward that the fuel needle have a fuel jet deflector at its pintletip. Such a fuel jet deflector is used to deflect the injected fuelstream towards the rim of a piston cavity. It is used in engines wherethe geometry of the cavity does not permit a sufficient length of thefree fuel jet if a normal pintle-type nozzle is used.

Instead of the fuel jet deflector at the fuel needle, a fuel jetdeflector could be provided in the piston cavity. This will beparticularly useful if there is not enough space in the piston for asufficiently deep cavity and if no fuel jet deflector is used at thenozzle. The fuel injection system of the invention may be provided witha needle valve either opening towards the inside or towards the outsidein the direction of the combustion chamber.

It is further possible to provide several nozzle holes in the injectionvalve instead of the fuel jet deflectors.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described with reference to theaccompanying drawings, in which FIGS. 1 to 3 are longitudinal sectionsof different variants of the fuel injection valve proposed by theinvention, FIG. 4a is a longitudinal section of a fuel needle providedwith a fuel jet deflector, FIG. 4b shows an injection valve with severalnozzle holes, FIGS. 5 and 6 show the injection system of the inventionwith a fuel jet deflector on the bottom of the piston cavity, FIG. 7gives another variant of the invention, the nozzle opening towards theoutside, and FIG. 8 is a force/lift diagram.

Elements of identical function have identical reference numbers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The fuel injection system shown in FIG. 1 essentially comprises aninjection valve or nozzle 1 injecting directly into a cylinder space 12,with a housing 1a, an electromagnet 2' with a solenoid 2, and a fuelneedle 3 with a pintle tip 4, the needle 3 acting as an armature for thesolenoid 2 and pressing against the spring 5. The fuel supply line ismarked 11. If the fuel needle 3 is directly operated by a solenoid 2,which itself is controlled via a control unit 10, the nonlinearity ofthe magnetic force F_(mag) vis-a-vis the linear spring force F_(F) maybe utilized to vary the velocity of the needle lift x. The magneticforce F_(mag) =C₁ /x² and the spring force F_(F) =C₂ *x+F₀. F₀, C₁, C₂are constants, x stands for the needle lift. In view of the quadraticgrowth of the lifting velocity due to the combined action of themagnetic force F_(mag) and the spring force F_(F), the needle 3 willremain for a longer time in the throttled region and the flow-reducingeffect of the pintle tip 4 is increased.

Due to the strongly nonlinear opening velocity of the needle it isfurther possible to reduce the needle, lift x necessary for the desiredflow regulation, such that a very small air gap h will suffice betweenthe fuel needle 3 and the solenoid 2. The very small air gap h willpermit the use of a very small solenoid 2. For practical purposes theopening velocity of the needle 3 should be as small as possible in orderto obtain the desired throttling effect of the pintle tip 4, which iseffective only in a very small range of the needle lift x, i.e., 0.05mm, approximately.

FIG. 2 shows a variant of the fuel injection valve 1 of the invention,with a single solenoid and two springs 5a and 5b which are placed onebehind the other. The first, weaker spring 5a will open even if onlyweak magnetic forces F_(mag) are applied by the solenoid 2. The secondspring 5b, which becomes effective when the lift hi released by thefirst spring 5a is surpassed, will open only if a strong magnetic forceF_(mag) is provided. After the first spring 5a has opened, a certainlength of time is required until the magnetic force F_(mag) growssufficiently strong to open the second spring 5b, during which time thefuel needle 3 will remain in the throttling position. The symbol h₂refers to the lift of the second spring 5b.

FIG. 3 presents a third variant of the invention with two solenoids 2aand 2b connected in series. The first solenoid 2a produces a certainforce F_(mag),1, which will lift the needle 3 to the throttling positionthrough the lift h₁. When the second solenoid 2b is additionallyenergized, the magnetic force F_(mag) will grow by F_(mag),2 of thesecond solenoid 2b, such that the nozzle will open completely. Bysuitably timing the operation of the solenoids 2a and 2b, the timing ofthe reduced flow may be adjusted as desired. As indicated in FIG. 3, thestepped movement of the fuel needle 3 may be supported by the additionaluse of serially placed springs 5a, 5b, in analogy to FIG. 2.

To obtain complete evaporation of the throttled fuel jet in thecombustion chamber, a fuel jet deflector 6 is advantageously used at thepintle tip 4. With the use of such a deflector the injected fuel streamis deflected towards the rim 9a of a cavity 9 provided in areciprocating piston 8 in the cylinder 13, as is shown in FIG. 4a.Instead of the fuel jet deflector 6 at the pintle tip 4, a fuel jetdeflector 7a, 7b may be provided in the piston cavity 9, as is shown inFIGS. 5 and 6. In the variant shown in FIG. 5 the fuel jet deflector 7adeflects the injected fuel along the bottom 8a of the piston cavity 9.The fuel jet deflector 7b indicated in FIG. 6 has a concave shape anddeflects the fuel in the direction of the cavity rim 9a. Instead ofusing fuel jet deflectors, the fuel injection valve 1 may be providedwith several nozzle holes 14, as is shown in FIG. 4b.

FIG. 7 presents another variant of an injection valve according to theinvention, in which the fuel needle 3' opens towards the outside in thedirection of the combustion chamber. The valve operates on the sameprinciple as described in FIGS. 2 and 3, with the exception that thedirection of the magnetic force F_(mag) and the spring force F_(F) isreversed. Again, variants with one or two series-connected solenoids 2a,2b and/or springs 5a, 5b are possible.

FIG. 8 shows a diagram in which the magnetic force F_(mag) and thespring force F_(F) are plotted against the needle lift x, the symbols Aand B referring to the needle in its closed and open position,respectively. The resultant force F_(res) acting on the fuel needle isthe difference of magnetic force F_(mag) and spring force F_(F), and isindicated by the hatched area in the diagram. The opening velocity isproportional to the resultant force F_(res) and will thus increaseapproximately quadratically as the magnetic force F_(mag) increases. Inthe initial region of the lift x indicated by the symbol a, i.e., in thefirst third, the needle opens at low velocity and the injected fuel isflow-reduced. In this initial region a the spring characteristic F_(F)is approximately parallel to a tangent t to the parabolic characteristicof the solenoid when the needle is in its closed position A. Beyond theinitial region a the velocity of the needle grows rapidly until the fullopening position B is assumed. The region of rapidly growing velocity ismarked b. The maximum resultant force F_(res),max and thus maximumopening velocity may amount to more than ten times the initial value.The minimum resultant force when the needle is closed is referred to asF_(res),min.

I claim:
 1. A fuel injection system for use in a self-igniting internalcombustion engine which defines a cylinder containing a reciprocatingpiston and which provides a combustion chamber, said fuel injectionsystem comprising an injection valve for direct fuel injection ofliquified gases into said combustion chamber, said injection valveincluding a housing defining a discharge nozzle, a fuel needle locatedin said housing and having a pintle tip that moves up to about 1 mmrelative to said nozzle to close said nozzle or to open said nozzle toprovide a throttled fuel flow therethrough, a spring located within saidhousing to bias said fuel needle into a closed position relative to saidnozzle, and an electromagnet means for directly applying a force on saidfuel needle to lift said fuel needle against said bias of said springand lift said fuel needle so as to open said nozzle, magnetic force fromsaid electromagnet means increasing quadratically with lift of saidneedle against a linearly increasing bias force of said spring,resulting in an approximately quadratically increasing opening velocityof said fuel needle in certain regions to permit small pintle lift,wherein said electromagnet means comprises a solenoid, wherein said fuelneedle includes a generally disc-shaped end portion which is spaced fromsaid solenoid, and wherein said spring extends within said solenoid toabut said disc-shaped end portion to bias said end portion in spacedrelation to said solenoid.
 2. A fuel injection system according to claim1, wherein said fuel needle has a maximum needle lift of smaller than0.5 mm.
 3. A fuel injection system according to claim 1, wherein acharacteristic of the spring force in a throttled region isapproximately parallel to a tangent to a characteristic of the magneticforce at a closing point of said fuel needle.
 4. A fuel injection systemaccording to claim 1, wherein the spring force acting on the fuel needleis provided by two serially placed springs with differentcharacteristics.
 5. A fuel injection system according to claim 1,wherein said electromagnet has at least two solenoids to be activatedindependently of each other for control of the magnetic force acting onsaid fuel needle.
 6. A fuel injection system according to claim 1,wherein said fuel needle has a fuel jet deflector at its pintle tip. 7.A fuel injection system according to claim 1, wherein a fuel jetdeflector is provided on a bottom of a cavity of said piston, in an areahit by an injected fuel jet of said injection valve.
 8. A fuel injectionsystem according to claim 1, wherein said fuel needle opens in thedirection of said combustion chamber.
 9. A fuel injection systemaccording to claim 1, wherein said injection valve is provided withseveral nozzle holes.